1) One problem is the one linked to in the (apparently dead) WP trac ticket. That has to do with declension and other noun forms not seen in English. But Spanish (the language of the OP) nouns don't decline. Moreover, if this were a declension problem, the screenshot would show the month name in the wrong case, not totally untranslated. So I don't think it has anything to do with the issue of declension, or really with date_l18n() at all.

2) I'm not sure I understand the problem that Andy's raising regarding dateboxes. I assume he's talking about bp_format_date, and in particular the line that hardcodes the date format. Given that this is just a display function, I'm not sure why it would have any effect on the information you can add with a datebox profile field. I just patched my local version with the attached patch 1068-1.patch and datebox selectors worked fine. BTW I think that the attached patch is a good idea, though again it's not really related to the OP's problem, which had to do with non-translation, not incorrect translation

3) My sense is that the OP's problem is this: buddypress-es_ES.mo had been installed, but not es_ES.mo (the regular WP language pack). Since date translations come from WP core, this would explain why the OP wasn't seeing date translations. I installed the Spanish language packs for BuddyPress and WP and dates seem to be working fine throughout.

I'm going to close this ticket because I don't really think there's an issue here. If I'm misunderstanding the root problem (possible!!), please reopen.

The former if you are looking for a specific format to be implemented. (It should be translatable, and yes, translators will realize this is a date.) But the latter is likely proper in this situation. The core date_format should be respected here, it's why it exists.